Activation of the MAP kinase/ERK Cascade by extracellular signals is believed to regulate a wide range of cellular processes, including transcription and cell proliferation. The objectives of this proposal are to study the biochemical and biological properties of the STE20- related protein kinases PAK1 and PAK2. The regulation of these kinases by small G proteins will be examined and the substrate specificity of these kinases will be determined. Isolated catalytic or regulatory domains will be introduced into cells to determine if they alter cell proliferation, influence the actin cytoskeleton (membrane ruffling, stress fibers), or activate the MAP kinase pathway alone or in response to growth factors. Finally, the subcellular distribution of these enzymes, which may help elucidate their functions, will be determined by immunofluorescence.